1. Field of the Invention
The invention relates to a conducting wire end portion joining method for conductively joining together end portions of conducting wires each made of a flat wire or the like. The invention relates also to a conducting wire end portion joining structure in which end portions of conducting wires are conductively joined together.
2. Description of Related Art
A stator of a motor includes a stator core made of laminated steel sheets each having an annular yoke, a plurality of teeth protruding radially inward from the yoke, and slots defined between the teeth that are adjacent to each other. The stator is formed by winding coils around the teeth while inserting the coils into the slots.
In winding wires for the coils, insulation coatings are formed around the conducting wires. Thus, insulation between the conducting wires in the coils is provided. Further, insulation between the stator core and the coils is provided by an insulator, such as an insulating bobbin or insulating paper, disposed between the teeth and the coils. Moreover, when, for example, distributed winding is employed as a winding configuration, phase-to-phase insulation between different phase coils is provided by an insulator, such as insulating paper.
A winding wire for forming a coil according to related art is manufactured as follows. A conducting wire made of, for example, a copper material is coated with a thermosetting enamel resin dissolved in a solvent, such that a coating layer having a thickness of several μm is formed around the conducting wire. Then, the coating layer is hardened by heat treatment. This process is repeated several times to form an enamel coating having a desired thickness. As the material of this enamel coating, an amide-imide resin, a urethane resin, an ester-imide resin, a polyimide resin, an oxide film, or the like is typically used. As the winding wire, a winding wire with a circular cross-section or a flat wire that is able to increase the coil space factor may be used.
As an actual method for forming coils around the teeth, the following method is employed. First, coils formed by winding conducting wires are fitted around the teeth. Then, end portions of the adjacent coils (winding wires) are connected to each other at coil ends by welding to form a continuous body of coils.
A method for conductively joining together end portions of coils (winding wires) according to related art will be briefly described with reference to FIGS. 4A to 4E.
First, as shown in FIGS. 4A and 4B, end portions Ma of winding wires M (flat wires) are brought into contact with each other. Each winding wire M includes a conducting wire W having a rectangular cross-sectional shape, and an insulation coating Z formed around the conducting wire W. Then, the insulation coatings Z on both of the end portions are stripped off to expose the conducting wires W of the end portions, as shown in FIG. 4C.
Next, the end portions of the conducting wires W are welded together by, for example, tungsten inert gas welding (TIG welding) having a welding temperature of approximately 500° C. Thus, the end portions of the conducting wires W are conductively joined together at a weld zone Y, as shown in FIG. 4D.
Finally, as shown in FIG. 4E, the conducting wires W of the end portions and the weld zone Y are coated with epoxy resin powder or the like to form an insulation coating Z′. In this way, the end portions of the winding wires are joined together to form a winding wire end portion joining structure S. It is necessary to strip off the insulation coatings Z as shown in FIG. 4C, for the following reason. If the insulation coatings Z are left on the end portions, the insulation coatings Z will carbonize because the welding temperature of welding is a high temperature of approximately 500° C. As a result, the carbonized insulation coatings Z are not appropriately connected to weld metal and the insulation coating Z′ that is ultimately formed.
Japanese Patent Application Publication No. 2013-93968 (JP 2013-93968 A) also describes a conducting wire end portion joining method. According to this method, in order to join end portions of winding wires together, first, insulation coatings of the end portions of the winding wires are stripped off to expose conducting wires, then the conducting wires are conductively connected together by welding, and then the conducting wires of the end portions and a weld zone are coated with insulating resin powder such that they are covered with the insulating resin powder.
The conducting wire end portion joining method according to the above-described related art requires three processes, that is, a process of stripping off the insulation coatings of the end portions of the conducting wires to be joined together, a welding process, and a process of forming an insulation coating at a joint portion at which the end portions are joined together. Thus, the method requires a lot of manufacturing time and laborious work.
Furthermore, as is also evident from FIG. 4E, the weld zone Y is formed at distal ends of the conducting wires W of the end portions of the winding wires M, and the insulation coating Z′ is formed such that the weld zone Y is covered with the insulation coating Z′. Thus, a height t1 of the joint portion, at which the conducting wires W are joined together, inevitably becomes high. Because the height of the joint portion at the coil ends becomes high, the physical size of the coils becomes large.